#define _CRT_SECURE_NO_WARNINGS 1
#include<iostream>
#include<vector>
using namespace std;

 struct TreeNode {
    int val;
    TreeNode* left;
    TreeNode* right;
    TreeNode() : val(0), left(nullptr), right(nullptr) {}
    TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
    TreeNode(int x, TreeNode* left, TreeNode* right) : val(x), left(left), right(right) {}
    
};

class Solution {
public:
    vector<int> ans;
    vector<int> postorderTraversal(TreeNode* root) {
        dfs(root);
        return ans;
    }
    void dfs(TreeNode* root)
    {
        if (root == nullptr)
            return;

        dfs(root->left);
        dfs(root->right);
        ans.push_back(root->val);
    }
};